Direct conversion, or Zero-IF (ZIF), radio receivers are well known in the industry and offer many benefits over heterodyne and other receiver architectures. In particular, the problem of image is entirely eliminated, and the IF or baseband filtering is simplified and can be easily integrated onto silicon devices.
FIG. 1 shows two signal graphs 10 and 20. These graphs illustrate a desired band 12 and an image 14, and show how the image 14 becomes noise after heterodyne mixing. In a “homodyne”, or ZIF receiver, ωLO=ω1, and so there is no image. This advantage for ZIF receivers comes at a cost: the down-converted signal now has content extending down to zero frequency. This requires that any analog processing have good noise properties at very low frequencies. This frequency range is where amplifier flicker noise is problematic.
There is a trend in the industry for increasing data rates to the mobile or wireless user, which translates to increased amplifier bandwidth requirements for the ZIF radio receiver.
ZIF radio receiver baseband processing is intended to provide good low frequency noise performance and high bandwidth.
Low frequency noise is usually dominated by the flicker noise performance of an analog amplifier. Flicker noise can be reduced by increasing the size of transistor devices in an amplifier, but this is traded-off with high frequency performance because the large size introduces large parasitic capacitances which reduce amplifier bandwidth. This loss of bandwidth can be reduced by increasing the dissipated power of the amplifier, but this is contraindicated by the need for lower power in wireless and mobile applications.
Improvements are desirable that allow good low frequency noise performance and high amplifier bandwidth at low dissipated power levels.